In gas turbine engines, fuel is delivered from a source of fuel to a combustion section where the fuel is mixed with air and ignited to generate hot combustion products defining working gases. The working gases are directed to a turbine section. The combustion section may comprise one or more stages, each stage supplying fuel to be ignited. It has been found that the production of NOx gases from the burning fuel can be reduced by providing fuel downstream from a main combustion zone.
A prior art method of delivering fuel to the downstream combustion zone of the combustion section is illustrated in U.S. Pat. No. 6,192,688. Fuel is delivered to a location downstream from the main combustion zone by fuel/air injection spokes extending radially into a secondary reaction zone. The fuel/air injection spokes include a plurality of fuel orifices for delivering fuel from a fuel manifold, and include a plurality of air orifices for delivering air from an air manifold. The fuel and air are ignited in the secondary reaction zone by hot products of combustion exiting the primary combustion zone.
Generally, prior art injectors for delivering fuel to downstream combustion zones have required substantial quantities of a diluent, such as air or steam, to be delivered with the fuel to achieve a required cooling of the outer wall of the injector. The delivery of such diluents with the fuel adds to the complexity, with associated costs, of the gas turbine engine. In addition, when air is used as a diluent, the compressor for supplying the air represents an additional parasitic loss that affects the overall plant efficiency.